You may recall that I gave a talk recently at a meeting called The Origins of the Expanding Universe in Flagstaff, Arizona. I put the slides up here. Well, the organizers have now put videos of the presentations online so you have the chance to see mine, warts and all.

I was relieved when I saw this on Youtube that the organizers were kind enough to edit out the embarrassing bit at the start when my laptop refused to talk to the data projector and I had to swap to another one. Sorting all that out seemed to take ages, which didn’t help my frame of mind and I was even more nervous than I would have been anyway given that this was my first public appearance after a rather difficult summer. Those are my excuses for what was, frankly, not a particularly good talk. But at least I survived. Better is the end of a thing than the beginning thereof.

For those of you interested in such things, here are the slides I used in my talk at the Origins of the Expanding Universe conference. I spoke about the events on and after 29th May 1919, when measurements were made during a total eclipse of the Sun that have gone down in history as vindicating Einstein’s (then) new general theory of relativity. I’ve written quite a lot about this in past years, including a little book and a slightly more technical paper. This was a relevant topic for the conference because it wasn’t until general theory of relativity was established as a viable theory of gravity that an explanation could be developed of Slipher’s measurements of galaxy redshifts in terms of an expanding Universe.

I’m grateful to George Ellis for sending me a link to a book review written by Freeman Dyson that appeared in a recent edition of the New York Review of Books. I was particularly interested to read the following excerpt about Arthur Stanley Eddington. I have been intrigued by Eddington since I wrote a book about his famous expeditions (to Principe and Sobral) in 1919 to measure the bending of light by the Sun as a test of Einstein’s general theory of relativity; I blogged about this on its ninetieth anniversary, by the way, in case anyone wants to read any more about it.

Although I read quite a lot about Eddington, not only during the course of researching the book but also afterwards, as there are many things about his character that fascinate me. He died long before I was born, of course, but whenever I meet someone who knew him I ask what they make of him. Not altogether surprisingly, opinions differ rather widely from one person to another as his character seems to have been extremely contradictory. He doesn’t seem to have been very good at small talk, but was nevertheless a much sought-after dining companion. He was a man of great moral integrity, but at times treated his colleagues (notably Chandrasekhar) rather shamefully. He was a brilliant astrophysicist, but got himself hooked on his peculiar Fundamental Theory which was a dead end. He remains an enigma.

Anyway, this is what Dyson has to say about him:

Eddington was a great astronomer, one of the last of the giants who were equally gifted as observers and as theorists. His great moment as an observer came in 1919 when he led the British expedition to the island of Principe off the coast of West Africa to measure the deflection of starlight passing close to the sun during a total eclipse. The purpose of the measurement was to test Einstein’s theory of General Relativity. The measurement showed clearly that Einstein was right and Newton wrong. Einstein and Eddington both became immediately famous. One year later, Eddington published a book, Space, Time and Gravitation, that explained Einstein’s ideas to English-speaking readers. It begins with a quote from Milton’s Paradise Lost:

Perhaps to move
His laughter at their quaint opinions wide
Hereafter, when they come to model heaven
And calculate the stars: how they will wield
The mighty frame: how build, unbuild, contrive
To save appearances.

Milton had visited Galileo at his home in Florence when Galileo was under house arrest. Milton wrote poetry in Italian as well as English. He spoke Galileo’s language, and used Galileo as an example in his campaign for freedom of the press in England. Milton had witnessed with Galileo the birth struggle of classical physics, as Eddington witnessed with Einstein the birth struggle of relativity three hundred years later. Eddington’s book puts relativity into its proper setting as an episode in the history of Western thought. The book is marvelously clear and readable, and is probably responsible for the fact that Einstein was better understood and more admired in Britain and America than in Germany.

As a student at Cambridge University I listened to Eddington’s lectures on General Relativity. They were as brilliant as his books. He divided his exposition into two parts, and warned the students scrupulously when he switched from one part to the other. The first part was the orthodox mathematical theory invented by Einstein and verified by Eddington’s observations. The second part was a strange concoction that he called “Fundamental Theory,” attempting to explain all the mysteries of particle physics and cosmology with a new set of ideas. “Fundamental Theory” was a mixture of mathematical and verbal arguments. The consequences of the theory were guessed rather than calculated. The theory had no firm basis either in physics or mathematics.

Eddington said plainly, whenever he burst into his fundamental theory with a wild rampage of speculations, “This is not generally accepted and you don’t have to believe it.” I was unable to decide who were more to be pitied, the bewildered students who were worried about passing the next exam or the elderly speaker who knew that he was a voice crying in the wilderness. Two facts were clear. First, Eddington was talking nonsense. Second, in spite of the nonsense, he was still a great man. For the small class of students, it was a privilege to come faithfully to his lectures and to share his pain. Two years later he was dead.

The 29th May 2009 is a very special day that should be marked by anyone interested in the theory of relativity as it is the 90th anniversary of one of the most famous experiments of all time.

On 29th May 1919, measurements were made during total eclipse of the Sun that have gone down in history as vindicating Einstein’s (then) new general theory of relativity. I’ve written quite a lot about this in past years, including a little book and a slightly more technical paper. I decided, though, to post this little piece that is based on an article I wrote for Firstscience.

The Eclipse that Changed the Universe

A total eclipse of the Sun is a moment of magic: a scant few minutes when our perceptions of the whole Universe are turned on their heads. The Sun’s blinding disc is replaced by ghostly pale tentacles surrounding a black heart – an eerie experience witnessed by hundreds of millions of people throughout Europe and the Near East last August.

But one particular eclipse of the Sun, eighty years ago, challenged not only people’s emotional world. It was set to turn the science of the Universe on its head. For over two centuries, scientists had believed Sir Isaac Newton’s view of the Universe. Now his ideas had been challenged by a young German-Swiss scientist, called Albert Einstein. The showdown – Newton vs Einstein – would be the total eclipse of 29 May 1919.

Newton’s position was set out in his monumental Philosophiae Naturalis Principia Mathematica, published in 1687. The Principia – as it’s familiarly known – laid down a set of mathematical laws that described all forms of motion in the Universe. These rules applied as much to the motion of planets around the Sun as to more mundane objects like apples falling from trees.

At the heart of Newton’s concept of the Universe were his ideas about space and time. Space was inflexible, laid out in a way that had been described by the ancient Greek mathematician Euclid in his laws of geometry. To Newton, space was the immovable and unyielding stage on which bodies acted out their motions. Time was also absolute, ticking away inexorably at the same rate for everyone in the Universe.

Sir Isaac Newton by Sir Godfrey Kneller
Courtesy of the National Portrait Gallery, London Sir Isaac Newton proposed the first theory of gravity.

For over 200 years, scientists saw the Cosmos through Newton’s eyes. It was a vast clockwork machine, evolving by predetermined rules through regular space, against the beat of an absolute clock. This edifice totally dominated scientific thought, until it was challenged by Albert Einstein.

In 1905, Einstein dispensed with Newton’s absolute nature of space and time. Although born in Germany, during this period of his life he was working as a patent clerk in Berne, Switzerland. He encapsulated his new ideas on motion, space and time in his special theory of relativity. But it took another ten years for Einstein to work out the full consequences of his ideas, including gravity. The general theory of relativity, first aired in 1915, was as complete a description of motion as Newton had prescribed in his Principia. But Einstein’s description of gravity required space to be curved. Whereas for Newton space was an inflexible backdrop, for Einstein it had to bend and flex near massive bodies. This warping of space, in turn, would be responsible for guiding objects such as planets along their orbits.

Royal Observatory Greenwich Albert Einstein and Arthur Eddington: the father of relativity and the man who proved him right.

By the time he developed his general theory, Einstein was back in Germany, working in Berlin. But a copy of his general theory of relativity was soon smuggled through war-torn Europe to Cambridge. There it was read by Arthur Stanley Eddington, Britain’s leading astrophysicist. Eddington realised that Einstein’s theory could be tested. If space really was distorted by gravity, then light passing through it would not travel in a straight line, but would follow a curved path. The stronger the force of gravity, the more the light would be bent. The bending would be largest for light passing very close to a very massive body, such as the Sun.

Unfortunately, the most massive objects known to astronomers at the time were also very bright. This was before black holes were seriously considered, and stars provided the strongest gravitational fields known. The Sun was particularly useful, being a star right on our doorstep. But it is impossible to see how the light from faint background stars might be bent by the Sun’s gravity, because the Sun’s light is so bright it completely swamps the light from objects beyond it.

Royal Observatory Greenwich Scientist’s sketch of the path of the vital 1919 eclipse.

Eddington realised the solution. Observe during a total eclipse, when the Sun’s light is blotted out for a few minutes, and you can see distant stars that appear close to the Sun in the sky. If Einstein was right, the Sun’s gravity would shift these stars to slightly different positions, compared to where they are seen in the night sky at other times of the year when the Sun far away from them. The closer the star appears to the Sun during totality, the bigger the shift would be.

Eddington began to put pressure on the British scientific establishment to organise an experiment. The Astronomer Royal of the time, Sir Frank Watson Dyson, realised that the 1919 eclipse was ideal. Not only was totality unusually long (around six minutes, compared with the two minutes we experienced in 1999) but during totality the Sun would be right in front of the Hyades, a cluster of bright stars.

But at this point the story took a twist. Eddington was a Quaker and, as such, a pacifist. In 1917, after disastrous losses during the Somme offensive, the British government introduced conscription to the armed forces. Eddington refused the draft and was threatened with imprisonment. In the end, Dyson’s intervention was crucial persuading the government to spare Eddington. His conscription was postponed under the condition that, if the war had finished by 1919, Eddington himself would lead an expedition to measure the bending of light by the Sun. The rest, as they say, is history.

The path of totality of the 1919 eclipse passed from northern Brazil, across the Atlantic Ocean to West Africa. In case of bad weather (amongst other reasons) two expeditions were organised: one to Sobral, in Brazil, and the other to the island of Principe, in the Gulf of Guinea close to the West African coast. Eddington himself went to Principe; the expedition to Sobral was led by Andrew Crommelin from the Royal Observatory at Greenwich.

Royal Observatory Greenwich British scientists in the field at Sobral in 1919.

The expeditions did not go entirely according to plan. When the day of the eclipse (29 May) dawned on Principe, Eddington was greeted with a thunderstorm and torrential rain. By mid-afternoon the skies had partly cleared and he took some pictures through cloud.

Meanwhile, at Sobral, Crommelin had much better weather – but he had made serious errors in setting up his equipment. He focused his main telescope the night before the eclipse, but did not allow for the distortions that would take place as the temperature climbed during the day. Luckily, he had taken a backup telescope along, and this in the end provided the best results of all.

After the eclipse, Eddington himself carefully measured the positions of the stars that appeared near the Sun’s eclipsed image, on the photographic plates exposed at both Sobral and Principe. He then compared them with reference positions taken previously when the Hyades were visible in the night sky. The measurements had to be incredibly accurate, not only because the expected deflections were small. The images of the stars were also quite blurred, because of problems with the telescopes and because they were seen through the light of the Sun’s glowing atmosphere, the solar corona.

Before long the results were ready. Britain’s premier scientific body, the Royal Society, called a special meeting in London on 6 November. Dyson, as Astronomer Royal took the floor, and announced that the measurements did not support Newton’s long-accepted theory of gravity. Instead, they agreed with the predictions of Einstein’s new theory.

Royal Observatory Greenwich The final proof: the small red line shows how far the position of the star has been shifted by the Sun’s gravity.

The press reaction was extraordinary. Einstein was immediately propelled onto the front pages of the world’s media and, almost overnight, became a household name. There was more to this than purely the scientific content of his theory. After years of war, the public embraced a moment that moved mankind from the horrors of destruction to the sublimity of the human mind laying bare the secrets of the Cosmos. The two pacifists in the limelight – the British Eddington and the German-born Einstein – were particularly pleased at the reconciliation between their nations brought about by the results.

But the popular perception of the eclipse results differed quite significantly from the way they were viewed in the scientific establishment. Physicists of the day were justifiably cautious. Eddington had needed to make significant corrections to some of the measurements, for various technical reasons, and in the end decided to leave some of the Sobral data out of the calculation entirely. Many scientists were suspicious that he had cooked the books. Although the suspicion lingered for years in some quarters, in the end the results were confirmed at eclipse after eclipse with higher and higher precision.

NASA In this cosmic ‘gravitational lens,’ a huge cluster of galaxies distorts the light from more distant galaxies into a pattern of giant arcs.

Nowadays astronomers are so confident of Einstein’s theory that they rely on the bending of light by gravity to make telescopes almost as big as the Universe. When the conditions are right, gravity can shift an object’s position by far more than a microscopic amount. The ideal situation is when we look far out into space, and centre our view not on an individual star like the Sun, but on a cluster of hundreds of galaxies – with a total mass of perhaps 100 million million suns. The space-curvature of this immense ‘gravitational lens’ can gather the light from more remote objects, and focus them into brilliant curved arcs in the sky. From the size of the arcs, astronomers can ‘weigh’ the cluster of galaxies.

Einstein didn’t live long enough to see through a gravitational lens, but if he had he would definitely have approved….

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